Our proposed research is to explore the neural mechanisms of complex-sound processing for specialized (biosonar) and common (communication) auditory functions. There are now a number of data indicating that the recognition of speech sounds in humans is based upon auditory perceptual mechanisms common to mammals. Our proposed research is very important for understanding auditory mechanisms in higher vertebrates, including humans, as well as in bats. Our ultimate goal is the complete understanding of both species-specific and common neural mechanisms for the processing of complex sounds. The mustached bat emits complex sounds for biosonar and communication. Its communication calls are quite different film its biosonar sounds, but are similar in spectral patterns to calls of other mammals, except for being high in frequency and relatively short in duration. We have demonstrated: (i) the processing of different types of biosonar information is parallel- hierarchical, (ii) the central auditory system creates neurons tuned to different types of information-bearing parameters (IBP's) characterizing biosonar signals, (iii) certain types of IBP's are extracted by neurons sensitive to different combinations of signal elements, (iv) different types of IBP's are systematically represented in separate cortical areas, (v) certain response properties of combination-sensitive neurons in the cortex are created by prethalamic auditory nuclei and art shaped by both thalamo-cortical ascending and cortico-thalamic descending systems, and (vi) the call-processing system overlaps with the biosonar-signal- processing system even in the auditory cortex. These findings are very important for understanding neural mechanisms for processing complex sounds in general. Our project I is to study further whether different cortical areas have neurons tuned to different types of IBP's found in the biosonar and communication sounds, whether these cortical areas are organized for a systematic representation of a particular IBP, and how tuning to a particular IBP is created. We will deliver acoustic stimuli mimicking natural sounds and information bearing elements (IBE's) in them and will study responses of single neurons in different cortical areas. In Project II, we will focus on the response properties of collicular, thalamic or cortical neurons which were well characterized with acoustic stimuli mimicking biosonar signals (pulse-echo pairs), and we will study how the response properties of thalamic or collicular neurons are influenced by electrical micro-stimulations of or micro-drug (local anesthetic or inhibitory-synaptic-transmitter agonist) injections into isotopic or anisotopic portions of cortical areas. Our proposed research will further contribute to the understanding of neural mechanisms for processing complex sounds, including speech sounds.